In commercial restaurants and institutional cooking places, it is common to have a powered venting or exhaust system to remove the cooking fumes from the kitchen for discharge through an exhaust duct to the outside atmosphere. A vent hood is typically located in the kitchen overlying a particular cooking appliance, such as a deep fat fryer, range, or griddle, and communicates with the exhaust duct which typically in turn rises upwardly from the hood through the building wall or roof at an elevation higher than the hood. A fan or blower in the duct work provides a negative pressure for forced ventilation of the kitchen air through the exhaust system, but because of the differential in elevation, even when the fan is not operating there nonetheless is a marked chimney effect created through the duct work.
A typical hazard of a commercial kitchen is the possibility of a fire, because of the collection of grease and other inflammable substances and the ever present heat and even open flames of the kitchen. Further, the exhaust duct system becomes a fire risk after continued use, without proper cleaning, because of the buildup of grease on the inside of the duct work. Accordingly, it is generally imperative that the ventilating system have a damper that can be closed to block the free "chimney" effect passage of air from the kitchen through the duct work to the atmosphere, and that appropriate controls deenergize the ventilator power fan unit. These precautions are necessary to preclude the possible spreading of a kitchen fire to other locations and in many respects, are required to achieve an acceptable fire rating under various testing laboratories such as the Underwriters' Laboratories, or of the National Sanitation Foundation, or under various building codes of the local area, city, or state.
One system thus far employed in automatic fire rated ventilators might include, for example, a damper which is normally closed by a spring and which is held open by a fusible link. Consequently, upon the link being melted by the heat of a proximate fire, the damper is slammed shut to block the exhaust vent passage. The difficulty in the continued use of such a system is that the damper is seldom opened and closed. Consequently, it can become bound by the buildup of grease or other dirt in the bearings or the spring can lose its snap because of heat or age, to the end that even with the fusible link removed the damper may not properly close. However, even if the damper did close successfully during a fire and even if no major damage occurred in the venting system, the damper safety device would not be workable until the damper was manually opened and a replacement fusible link put in place.
Another type of safety control that is used commonly to automatically trip the damper in the event of a fire is electrically operated. Under such circumstances, a fire sensing means triggers an electric signal to a solenoid which either releases the damper to allow the same to be mechanically shifted to its closed position or that actually electrically shifts the damper to its closed position. In any regard, a problem with this type control is that frequently the electric power to the facility is interrupted during a fire to render the electrically actuated safety damper unusuable thereafter. Furthermore, even though the damper might successfully be closed by the electrically actuated controls, nonetheless it must generally thereafter be manually opened after the fire is put out. Further, unless such an electrically actuated system is tested for operational certainty on a regular basis, again after periods of nonuse the same may or may not be in proper working condition; and the unsuspected user may not be the wiser until after the fact and the fire was allowed to burn without the damper properly closing.
Various patents which illustrate the systems noted above are as follows: Gaylord U.S. Pat. No. 2,813,477; Graswich et al. U.S. Pat. No. 2,961,941; Gaylord U.S. Pat. No. 3,055,285; Gaylord U.S. Pat. No. 3,207,058; Gaylord U.S. Pat. No. 3,247,776; Gaylord U.S. Pat. No. 3,611,909; Gaylord U.S. Pat. No. 3,785,124.